1. Field of the Invention
This invention concerns a device for fixed positioning of a pallet on a work table.
2. Discussion of the Prior Art
German patent 3,923,574 discloses a device for changing pallets where a pallet changer is arranged in front of the work table that belongs with the machine. It has a motor-driven pallet carrier that can be raised and lowered and rotated about its vertical axis for replacing a pallet on the work table with a pallet in a holding position. The pallet is secured on the work table by means of several studs that are provided on each pallet and are secured in the work table by means of a hydraulic clamping unit. One disadvantage of this device is that a separate hydraulic drive is needed for operation of the pallet clamping device. The pallet changing drive and the hydraulic drive of the clamping device must be controlled separately and must be coordinated in a suitable manner. Therefore, a complex control system is necessary for the actuation and adjustment of the two separate drives.
Another device for automatic changing of pallets is disclosed in European patent application 521,316. That structure permits automatic and accurate positioning of the workpiece pallet on the pallet carrier. With this device, the clamping mechanism has two puller and pusher mechanisms that are under a pre-tension with respect to each other by means of compression springs and have wedges on their facing sides. The bolts attached to the lower side of each pallet have lateral key grooves in which the wedges of the two puller and pusher mechanisms engage so they lock together. At least one of the puller and pusher mechanisms has a lateral recess in which a control element engages. Several shafts are mounted in the pallet carrier and each shaft has a control element at one end and can be rotated by the vertically movable pallet carrier by means of an actuating element at the other end. The vertical movement of the pallet changer is converted by mechanical actuating and control elements into a horizontal movement of the puller and pusher mechanisms to lock and release the pallet engaging clamps. Thus, no separate drive is needed to operate the pallet clamping device and the corresponding control elements can also be eliminated. However, a disadvantage of the known device is the relatively complicated design due to the fact that two shafts each having their own actuating and control elements must be provided.